Wavelength division multiplexing (WDM) optical communication systems have been deployed to increase capacity of existing optical fiber networks. In a WDM system, multiple optical signals having different wavelengths are combined onto a single optical fiber. After traveling through the fiber, the signals are then separated according to wavelength, and subject to further processing.
Most deployed fiber is silica-based, and typically has a relatively narrow, low absorption band centered about 1550 nm. Accordingly, in order to increase capacity further IN a WDM system, optical signal wavelengths are spectrally spaced close to one another. For example, early WDM systems had signal or channel spacings of about 100 GHz, but as capacity needs increased, later generation WDM systems were developed having narrower channel spacings of 50 GHz. Capacity requirements continue to increase, and even narrower channel spaced systems having spacing of 25 GHz or less are expected.
Each optical signal or channel in a WDM system is generated by a laser, typically, a semiconductor chip, which outputs light at one of the WDM channel wavelengths. Temperature variations can cause the wavelength of light output from the chip to vary. Accordingly, the laser chip temperature is tightly controlled. In addition, various wavelength locking schemes have been developed to keep the output wavelength locked to the desired channel wavelength. Such schemes are described, for example, in U.S. Pat. Nos. 5,875,273 and 5,943,152, incorporated by reference herein.
In conventional laser wavelength locking schemes, a portion of light emitted by a laser is supplied to a filter having an associated transmission spectrum that permits appropriate feedback circuitry to determine when the laser is locked to a desired wavelength. A thermo electric cooler (TEC) coupled to the feedback circuit controls the temperatures of the laser, so that the laser continues to output light at that wavelength. However, laser wavelength can drift with over extended periods of time. Moreover, the laser and the filter are often packaged within a common housing (often a hermetic “butterfly” package), and non-linear temperature distributions in the package are believed to cause refractive index changes in the filter, as well as thermal expansion of the filter and other optics in the package. In addition, these thermal variations are believed to change the physical dimensions of the laser chip. As a result, the output wavelength can change, even if conventional laser locking techniques are employed. In spectrally dense WDM system with narrow channel spacings, such channel drift can lead to one channel interfering with another, resulting in an unacceptable loss of data.